Semiconductor device and a method of increasing a resistance value of an electric fuse

ABSTRACT

Provided is a semiconductor device having an electric fuse structure which receives the supply of an electric current to be permitted to be cut without damaging portions around the fuse. An electric fuse is electrically connected between an electronic circuit and a redundant circuit as a spare of the electronic circuit. After these circuits are sealed with a resin, the fuse can be cut by receiving the supply of an electric current from the outside. The electric fuse is formed in a fine layer, and is made of a main wiring and a barrier film. The linear expansion coefficient of each of the main wiring and the barrier film is larger than that of each of the insulator layers. The melting point of each of the main wiring and the barrier film is lower than that of each of the insulator layers.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese patent applicationNo. 2006-256226 filed on Sep. 21, 2006, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device which receivesthe supply of an electric current so as to be permitted to increase theresistance of the device itself, and a method of increasing theresistance of an electric fuse.

Hitherto, there has been used a fuse which receives the supply of anelectric current to be permitted to increase the resistance of the fuseitself. In the present specification, such a fuse is called an electricfuse. The electric fuse is set inside an insulator layer. In thespecification, a structure having an insulator layer and an electricfuse is called an electric fuse structure. In the specification, anincrease in the resistance of an electric fuse is, for example, aphenomenon that the value of an electric current flowing into theelectric fuse becomes small, that is, the electric fuse turns into astate that the fuse has a higher resistance than before, or a phenomenonthat the flow of an electric current between two elements connected toboth ends of the electric fuse stops completely, that is, the electricfuse is cut or melted/cut, or the resistance of the electric fusebecomes infinite. Examples of the electric fuse described in thespecification include a fuse for making the use of an electric circuitimpossible, a fuse which is used in an analog device or the like toadjust the voltage of the device, and a fuse which is used as a tag forleaving the hysteresis of a process, a test result or the like.

[Patent Document 1] Pamphlet of WO 97/12401

[Patent Document 2] U.S. Pat. No. 5,969,404

[Patent Document 3] U.S. Pat. No. 6,323,535

[Patent Document 4] U.S. Pat. No. 6,433,404

[Patent Non-document 1] V. Klee et al., “A 0.13 μm logic based embeddedDRAM technology with electrical fuses, Cu interconnect in SiLk™, sub-7ns access and its extension to the 0.10 μm generation”, IEDM Conference(2001).

SUMMARY OF THE INVENTION

Increases in the resistance of conventional electric fuses are realizedby an electromigration phenomenon. For this reason, in some cases, it isnecessary to supply a large electric current to an electric fuse. Insuch cases, a structure around the electric fuse may be damaged by heatgenerated from the fuse.

In light of the above-mentioned problems, the present invention has beenmade. Thus, an object of the invention is to provide a semiconductordevice which is permitted to increase the resistance of the deviceitself without damaging any surrounding structure, and a method ofincreasing the resistance of an electric fuse.

An aspect of the present invention is a semiconductor device comprisingan insulator layer and an electric fuse formed in the insulator layer.The electric fuse has a larger linear expansion coefficient than that ofthe insulator layer, and further has a lower melting point than that ofthe insulator layer.

According to this structure, the resistance of the electric fuse can beincreased even if the value of an electric current supplied to theelectric fuse is small. Accordingly, the amount of heat generated fromthe electric fuse is small. As a result, a structure around the electricfuse is prevented from being damaged.

Another aspect of the invention is a semiconductor device comprising asemiconductor substrate, a gate electrode formed over the semiconductorsubstrate, an interlayer dielectric covering the gate electrode, a finelayer formed over the interlayer dielectric, a semiglobal layer formedover the fine layer, a global layer formed over the semiglobal layer,and an electric fuse formed in at least one selected from the finelayer, the semiglobal layer, and the global layer.

According to this structure, when an electric current is supplied to theelectric fuse, the distance over which heat generated from the electricfuse reaches the semiconductor substrate is large; therefore, theresistance of the electric fuse can be increased without damaging thesemiconductor substrate.

Still another aspect of the invention is a semiconductor devicecomprising an insulator layer, and an electric fuse which is formed inthe insulator layer, and has a meandering shape comprising a linearportion and a bent portion, wherein the distance between moieties nearthe bent portion is smaller than the distance between moieties otherthan the moieties near the bent portion.

According to this structure, heat from a central portion of the electricfuse does not diffuse outside easily since the electric fuse ismeandering. Therefore, a structure around the electric fuse isrestrained from being damaged by heat generated from the electric fuse.Moreover, a time required for an increase in the resistance of theelectric fuse can be shortened since a large amount of heat is locallygiven only to the bent portion.

A different aspect of the invention is a method of increasing theresistance of an electric fuse wherein an electric current is suppliedto the electric fuse which is any one of the above-mentioned electricfuses. In this way, the electric fuse is melted and is further cracked.Thereafter, apart of the melted electric fuse is absorbed into the crackby use of a capillary phenomenon. As a result, a discontinuous portionis formed in the electric fuse. According to this method, an electricfuse can be cut by a smaller electric current than that given to anelectric fuse in any conventional method of using electromigration tocut the electric fuse.

A further different aspect of the invention is a method of increasingthe resistance of an electric fuse comprising the steps of: supplying anelectric current to the electric fuse which is any one of theabove-mentioned electric fuses, thereby making the electric fuse narrowby use of pinch effect; and then stopping the supply of the electriccurrent, thereby forming a cavity in the electric fuse by use ofretaining force of the electric fuse. According to this method, anelectric fuse can be cut by a smaller electric current than that givento an electric fuse in the above-mentioned method of cutting theelectric fuse by use of a capillary phenomenon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a structure of an electroniccircuit to which an electric fuse of an embodiment of the invention isfitted.

FIG. 2 is a view illustrating a structure of the whole of asemiconductor device wherein an electric fuse structure of theembodiment is formed.

FIG. 3 is a schematic view illustrating the electric fuse of theembodiment which has a meandering shape.

FIG. 4 is a sectional view taken on line IV-IV in FIG. 3.

FIG. 5 is a schematic view illustrating the electric fuse of theembodiment which is made only of a liner portion.

FIG. 6 is a sectional view taken on line VI-VI in FIG. 5.

FIG. 7 is a schematic view illustrating another example of the electricfuse of the embodiment which has a meandering shape.

FIG. 8 is a photograph showing a state that linear portions of anelectric fuse of the embodiment which has a meandering shape contacteach other by leakage or solid dissolution.

FIG. 9 is a view illustrating a basic example of the electric fusestructure of the embodiment.

FIG. 10 is a first different example of the electric fuse structure ofthe embodiment.

FIG. 11A is a second different example of the electric fuse structure ofthe embodiment.

FIG. 11B is a third different example of the electric fuse structure ofthe embodiment.

FIG. 12A is a fourth different example of the electric fuse structure ofthe embodiment.

FIG. 12B is a fifth different example of the electric fuse structure ofthe embodiment.

FIG. 13 is a sixth different example of the electric fuse structure ofthe embodiment.

FIG. 14A is a seventh different example of the electric fuse structureof the embodiment.

FIG. 14B is an eighth different example of the electric fuse structureof the embodiment.

FIG. 15 is a ninth different example of the electric fuse structure ofthe embodiment.

FIG. 16A is a tenth different example of the electric fuse structure ofthe embodiment.

FIG. 16B is an eleventh different example of the electric fuse structureof the embodiment.

FIG. 17 is a twelfth different example of the electric fuse structure ofthe embodiment.

FIG. 18A is a thirteenth different example of the electric fusestructure of the embodiment.

FIG. 18B is a fourteenth different example of the electric fusestructure of the embodiment.

FIG. 19 is a fifteenth different example of the electric fuse structureof the embodiment.

FIG. 20A is a sixteenth different example of the electric fuse structureof the embodiment.

FIG. 20B is a seventeenth different example of the electric fusestructure of the embodiment.

FIG. 21 is a view for explaining the direction of force acting on theelectric fuse which is the basic example of the embodiment when anelectric current flows into this electric fuse.

FIG. 22 is a view for explaining a state that the electric fuse of thebasic example swells.

FIG. 23 is a top view illustrating a first state of the electric fuse ofthe basic example when it is cut.

FIG. 24 is a sectional view taken on line XXIV-XXIV in FIG. 23.

FIG. 25 is a top view illustrating a second state of the electric fuseof the basic example when it is cut.

FIG. 26 is a sectional view taken on line XXVI-XXVI in FIG. 25.

FIG. 27 is a top view illustrating a third state of the electric fuse ofthe basic example when it is cut.

FIG. 28 is a sectional view taken on line XXVIII-XXVIII in FIG. 27.

FIG. 29 is a top view illustrating a fourth state of the electric fuseof the basic example when it is cut.

FIG. 30 is a sectional view taken on line XXX-XXX in FIG. 29.

FIG. 31 is a top view illustrating a fifth state of the electric fuse ofthe basic example when it is cut.

FIG. 32 is a sectional view taken on line XXXII-XXXII in FIG. 31.

FIG. 33 is a photograph (of a cross section) showing a state that anelectric fuse is absorbed into a crack formed in an insulator layer inan electric fuse structure.

FIG. 34 is a photograph (of a top face) showing the state that theelectric fuse is absorbed into the crack formed in the insulator layerin the electric fuse structure.

FIG. 35 is a view illustrating an electric current pulse as an improperpulse, and an electric current pulse as a proper pulse.

FIG. 36 is a photograph showing an electric fuse cut by an electriccurrent pulse as an improper pulse, and an electric fuse cut by anelectric current pulse as a proper pulse.

FIG. 37 is a graph showing a relationship between rise time of electriccurrent pulses and the ratio of the resistance of an electric fuse afterthe fuse is cut to that of the electric fuse before the fuse is cut.

FIG. 38 is a top view illustrating an example of the position of a cutportion of an electric fuse made only of a linear portion.

FIG. 39 is a chart wherein positions of cut portions of plural electricfuses each made only of a linear portion are plotted.

FIG. 40 is a view for explaining an electric fuse structure wherein acentral portion is selectively to be cut.

FIG. 41 is a photograph showing an electric fuse structure wherein acentral portion was selectively cut.

FIG. 42 is a view illustrating the distance between linear portions.

FIG. 43 is a view illustrating a state that linear portionsshort-circuit through a cut piece.

FIG. 44 is a view illustrating an electric fuse structure having aconstruction for preventing linear portions from short-circuiting.

FIG. 45 is a view for explaining a method of cutting an electric fuse byuse of pinch effect.

FIG. 46 is a photograph showing an electric fuse cut by pinch effect.

FIG. 47 is a graph of a relationship between time and the distancebetween a moiety having a temperature of 600° C. when the temperature ofan electric fuse was kept at 1200° C. and the electric fuse.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the attached drawings, embodiments of thesemiconductor device according to the present invention and the methodof increasing the resistance of an electric fuse according to theinvention will be described hereinafter.

Embodiment 1

An electric fuse of an embodiment 1 of the present invention is not anyelectric fuse formed in the same layer in which a gate electrode isformed, as in the prior art. The electric fuse of the embodiment 1 isformed in a fine layer in a multi-layered structure including the finelayer, a semiglobal layer and a global layer in a semiconductor device.Therefore, the electric fuse is prevented from damaging itssemiconductor substrate.

According to the structure of the semiconductor device of the embodiment1, other elements, such as a transistor for controlling the flow of anelectric current for increasing the resistance of the fuse, can bearranged in a space from the semiconductor substrate to the electricfuse; therefore, it is possible to make small the occupation area ofelements arranged in a direction parallel to a main surface of thesemiconductor substrate of the semiconductor device.

The increase in the resistance of the electric fuse of the embodiment 1is realized not by any electromigration phenomenon but a capillaryphenomenon. Accordingly, the resistance of the electric fuse can beincreased only by causing a relatively small electric current to flowinto the electric fuse. As a result, a structure around the electricfuse is prevented from being damaged. Moreover, the time necessary foran increase in the resistance of the electric fuse can be largelyshortened.

In the embodiment 1, the electric fuse is a member for separating aredundant circuit and any other circuit electrically from each other.However, the usage of the electric fuse of the invention is not limitedthereto. The electric fuse of the invention can be applied to anyarticle as long as the article is an article having a resistance thatcan be increased by receiving the supply of an electric current. The rawmaterial of the electric fuse is suitably a metal or a metal compound.However, the raw material of the electric fuse of the invention is notlimited thereto as long as a resistance-increasing method that will bedescribed below can be applied to the raw material.

First, the electric fuse structure of the embodiment 1 is specificallydescribed herein. As illustrated in FIG. 1, the electric fuse (electricfuse 10) of the embodiment 1 is set inside a semiconductor device, andis connected to a power source electrode VDD and an earth electrode VSSso as to be present therebetween. A resistor 60 is arranged between aterminal 10 a of the electric fuse 10 and the power source electrodeVDD, and a resistor 70 is arranged between a terminal 10 b of theelectric fuse 10 and the earth electrode VSS. A transistor 40 and adecision circuit 50 are connected to a wiring between the resistor 70and the terminal 10 b. The decision circuit 50 is a circuit fordetecting whether or not the resistance of the electric fuse 10 turnsinto a predetermined value or more. An inverter circuit 30 is connectedto the gate electrode of the transistor 40. In accordance with anelectric signal given from the inverter circuit 30 to the transistor 40,an electric current flows from the power source electrode VDD throughthe electric fuse 10 to the earth electrode VSS. Accordingly, in themethod of increasing the resistance of the electric fuse 10 in theembodiment 1, whether or not the resistance of the electric fuse isincreased can be controlled in accordance with an electric signal givento the transistor 40 from the outside. Whether or not the resistance ofthe electric fuse 10 is over the predetermined value is decided by thedecision circuit 50.

Next, the structure of the semiconductor of the embodiment 1 isdescribed herein with reference to FIG. 2. The semiconductor device ofthe embodiment 1 has plural stacked metal wiring layers. The metalwiring layers are named M1, M2, . . . . . . M8 and M9, respectively, inthe order from the side of a semiconductor substrate SC upwards. Themetal wiring layers are connected to each other through vias. The viasare named V1, V2, . . . . . . , V7 and V8, respectively, in the orderfrom the side of the semiconductor substrate SC upwards.

Out of the layers including the metal wiring layers M1, M2, . . . . . .M8 and M9, and the vias V1, V2, . . . . . . , V7 and V8, layerspositioned at a lower side are called a fine layer 100, and layerspositioned at an upper side are called a global layer 300. The layerspositioned between the fine layer 100 and the global layer 300 arecalled a semiglobal layer 200.

The metal wiring layers in the fine layer 100 each have the smallestwiring width and thickness among the metal wiring layers constitutingthe semiconductor device. The metal wiring layers in the semigloballayer 200 each have a larger wiring width and a larger thickness thanthose of the metal wiring layers in the fine layer 100. The metal wiringlayers in the global layer 300 each have a larger wiring width and alarger thickness than those of the metal wiring layers in the semigloballayer 200. Examples of dimensions of the fine layer 100, the semigloballayer 200 and the global layer 300 are shown in Table 1.

TABLE 1 Wiring width(μm) Wiring thickness(nm) Fine layer 0.12 200Semiglobal layer 0.3 400 Global layer 0.6 1000

The dimensions of the fine layer 100, the semiglobal layer 200 and theglobal layer 300 are varied in accordance with the kind of thesemiconductor device, and the material of the wirings. Accordingly,Table 1 shows a mere example of a relationship between the dimensions ofthe three layers.

In a conventional semiconductor device, a wiring layer equivalent to agate electrode layer GA covered with an interlayer dielectric (TEOS:tetraethyl ortho silicate glass) CA shown in FIG. 2 is partially used asan electric fuse. For this reason, when a large electric current issupplied to the electric fuse so as to make the resistance of apredetermined portion of the electric fuse high, a semiconductorsubstrate of the conventional device and a surrounding portion thereofmay be damaged by heat generated from the electric fuse. Against thismatter, in the embodiment 1, the electric fuse 10 is arranged near themetal wiring layers M1 to M5 in the fine layer 100.

The metal wiring layers M1 to M5, which constitute the fine layer 100,are formed in accordance with a single rule for plural layers(generally, the number of the layers is from about 4 to 6), this matterbeing different from rules for the metal wiring layers M6 and M7, whichconstitute the semiglobal layer 200, and for the metal wiring layers M8and M9, which constitute the global layer 300. Therefore, the electricfuse 10 can be formed in any one of the layers in fine layer 100. Forexample, the electric fuse 10 can be formed near the metal wiring layerM5, which is formed at a position farthest from the semiconductorsubstrate SC.

Accordingly, when an electric current is supplied to the electric fuse10, heat generated from the electric fuse 10 is prevented from producingan adverse effect onto the semiconductor substrate SC. Even if theelectric fuse 10 is formed in the semiglobal layer 200 or the globallayer 300, the electric fuse 10 can be prevented from producing anadverse effect onto the semiconductor substrate SC. In other words, evenif the electric fuse 10 is formed in any one of the layers in the finelayer 100, the semiglobal layer 200 and the global layer 300, or theelectric fuse 10 and one or more electric fuses equivalent thereto areformed in any two or all of these layers, the electric fuse 10 can beprevented from producing an adverse effect onto the semiconductorsubstrate SC.

In the semiconductor device of the embodiment 1, the metal wiring layerwhich has a low resistance is used as the electric fuse 10. Thus, evenif the value of the electric current supplied to the electric fuse 10 issmall, the resistance of the electric fuse 10 can be increased.

FIGS. 3 and 4 are a top view and a sectional view of the electric fuse10 of the embodiment 1 and a portion around the fuse 10, respectively.The electric fuse 10 of the embodiment 1 has a meandering shape composedof linear portions 10 d and bent portions 10 c. The electric fusestructure of the embodiment 1 may have an electric fuse 10 made only ofa linear portion, as illustrated in FIGS. 5 and 6. However, when theelectric fuse 10 having a meandering shape is compared with any electricfuse which is made only of a linear portion and has the same length asthe meandering electric fuse 10, the meandering electric fuse 10 has anadvantage that even if the value of an electric current supplied to thefuse 10 is small, the resistance of the fuse 10 can be made larger.

As illustrated in FIGS. 3 to 6, in the electric fuse structure of theembodiment 1, the electric fuse 10 is surrounded by the metal wiringlayers M1 to M5 and the vias V1 to V4, which are each made of anelectroconductive material. The metal wiring layers M1 to M5 and thevias V1 to V4 illustrated in FIGS. 3 to 6 are each anelectrically-floating, electroconductive layer, which is electricallyinsulated from the other electroconductive layers. Accordingly, even ifthe electric fuse 10 melts out to leak into one or more of the insulatorlayers around the fuse 10, the metal wiring layers M1 to M5 and the viasV1 to V4 prevent the leaking fuse from producing an adverse effect ontoany different electronic circuit.

The electric fuse 10 of the embodiment 1 may have a structure asillustrated in FIG. 7. Specifically, the number of bent portions 10 cand that of linear portions 10 d are not each limited to any specificnumerical value.

FIG. 8 is a photograph showing a state that an example of the electricfuse 10 was actually cut. It can be understood from FIG. 8 that when theelectric fuse 10, which is meandering, is cut, its portions adjacent toeach other are brought into contact with each other to generate leakageand further a portion below the electric fuse 10 is cracked by anexpansion in the volume of the portion converted to a solid solution. Inother words, it can be understood that only an idea that the electricfuse 10 is meandered does not make it possible to increase theresistance of the electric fuse 10 while the electric fuse 10 isprevented from producing an adverse effect onto a structure around thefuse 10.

Consequently, the electric fuse 10 of the embodiment 1 has a structureillustrated in FIG. 9 in order to increase the resistance of theelectric fuse 10 while the electric fuse 10 is prevented from producingan adverse effect onto a structure around the fuse 10.

As illustrated in FIG. 9, the electric fuse 10 is made of a main wiring1 and a barrier film 3 covering the lower face of the main wiring 1 andboth side faces of the wiring 1. The electric fuse 10 extends inside atrench 2 a made in the insulator layer 2 and in parallel to the mainsurface of the semiconductor substrate SC. The electric fuse 10 and theinsulator layer 2 are covered with an insulator layer 4. An insulatorlayer 5 is formed on the insulator layer 4.

The main wiring 1 is made of a metal layer or a metal compound layer,and has a lower melting point than the insulator layer 2, the insulatorlayer 4 and the insulator layer 5 each have. The barrier film 3 is ametal layer or a metal compound layer, or has a structure wherein theselayers are stacked. The melting point of the barrier film 3 is higherthan that of the main wiring 1 and lower than those of the insulatorlayers 2 and 4. Furthermore, the linear expansion coefficient of themain wiring 1 is larger than that of the barrier film 3, and the linearexpansion coefficient of the barrier film 3 is as large as or largerthan that of each of the insulator layers 2, 4 and 5.

In the semiconductor device of the embodiment 1, the main wiring 1 ismade of a copper film, and the barrier film 3 is a tantalum film. Theinsulator layers 2 and 5 are each a SiOC film, which is a low-k filmhaving a dielectric constant of 3 or less, and the insulator layer 4 isa SiN film. However, the materials of the main wiring 1, the barrierfilm 3 and the insulator layers 2, 4 and 5 are not limited to theabove-mentioned materials as long as the materials satisfy theabove-mentioned relationships about the linear expansion coefficientsand the melting points. For example, the insulator layer 4 may be asilicon nitride film (SiN film). The material of the main wiring 1 maybe Al, Cu, Ta, Ti or W, as shown in Table 2.

TABLE 2 Linear expansion coefficient(10⁻⁶/K) Melting 300K 600K 800K1000K point(° C.) Al 23.2 28.4 34 — 660.4 Cu 16.6 18.9 20.3 22.4 1084.5Ta 6.3 — — 7.3 2996 Ti 8.7 10.4 11.1 11.5 1675 W 4.5 4.7 5 5.2 3387Oxide films or 0.5 to — — — About nitride films 10 1000 to used in the1600 field of semiconductors

The electric fuse structure of the invention is not limited to thestructure illustrated in FIG. 9, and may be a structure shown in each ofFIGS. 10 to 20B. The structures shown in FIGS. 10 to 20B basically havea structure similar to the electric fuse structure illustrated in FIG.9; therefore, the same reference number is attached to each of membersor parts common to each other in these structures, and descriptionthereof is not repeated. FIGS. 11A, 12A, 14A, 16A, 18A and 20Acorrespond to FIGS. 11B, 12B, 14B, 16B, 18B and 20B, respectively. Whichof a structure illustrated in FIG. 11A and a structure illustrated inFIG. 11B is formed depends on a used production process. Consequently,in one device, there may be formed both of any one of the structuresillustrated in FIGS. 11A, 12A, 14A, 16A, 18A and 20A and a structurecorresponding thereto out of the structures illustrated in FIGS. 11B,12B, 14B, 16B, 18B and 20B.

In a structure illustrated in FIG. 10, an insulator layer 4 is composedof an insulator layer 4 a and an insulator layer 4 b. The insulatorlayer 4 is a SiCO layer, and the insulator layer 4 b is a SiCN layer.

In each of the structures illustrated in FIGS. 11A, 11B, 12A and 12B, abarrier film 3 has a three-layer structure. The three-layer structure iscomposed of a Ta film 3 a formed on side faces of a trench 2 a, a TaNfilm 3 b formed on inner side faces of the Ta film 3 a, and a Ta film 3c formed on inner side faces of the TaN film 3 b and the bottom face ofthe trench 2 a.

In the structures illustrated in FIGS. 13 to 20B, a metal cap film 9made of CoW, COWP, CoP or COPB is formed on a main wiring 1. Theelectric resistance of the metal cap film 9 is higher than that of themain wiring 1. Accordingly, when the metal cap film 9 is made on themain wiring 1, a larger heat is generated than heat generated from onlythe main wiring 1. In short, the resistance of the electric fuse 10 canbe increased in a shorter time. The metal cap film 9 may be formed onthe barrier film 3. The metal cap film 9 is formed on the entire upperface of the main wiring 1 so as to have a function of preventing thegeneration of electromigration of the main wiring 1. In the embodiment1, the metal cap film 9 made of CoW, CoWP, CoP or CoPB is described asan example of the cap film. However, any film may be formed on the mainwiring 1 as long as the film has a higher electric resistance than thatof the main wiring 1.

In the structures illustrated in FIGS. 17 to 20B, the insulator layer 4is not formed. In this case, a crack 6 is formed in an insulator layer5.

The following will describe the effect generated when the resistance ofthe electric fuse of the embodiment 1 increases, in particular, theeffect generated when the electric fuse is cut.

First, table 3 is used to describe, herein, the volume expansioncoefficient of the metal which constitutes the main wiring 1 in theembodiment 1 when the metal is liquefied.

TABLE 3 Density at room Density of Reference: temperature liquefiedmelting point (g/cm³) metal (g/cm³) (° C.) Aluminum 2.69 2.5(800° C.) 660.4 Copper 8.93 7.8(1200° C.) 1084.5 Iron 7.86 7.1(1550° C.) 1535

It can be understood from Table 3 that the density of each of the metalsis smaller after liquefied than before liquefied. This matterdemonstrates that the volume of each of the metals after it is liquefiedincreases from that of the metal before it is liquefied. As shown inTable 3, the volume expansion coefficients of the metals based onliquefaction are as follows: the volume expansion coefficient of Al is8% (2.69/2.5=1.08); that of copper is 14% (8.93/7.8=1.14); and that ofiron is 11% (7.86/7.1=1.11). It can be therefore understood that thevolume expansion coefficient of copper is the highest among aluminum,copper and iron.

With reference FIGS. 21 and 22, the effect generated when the resistanceof the electric fuse 10 increases, in particular, the effect generatedwhen the electric fuse 10 is cut is described, considering theabove-mentioned matters.

In an electric fuse 10 illustrated in FIG. 21, an electric current flowsalong a direction perpendicular to the paper surface, that is, along adirection in which a main wiring 1 extends, whereby Joule heat isgenerated in the main wiring 1. Thus, the temperature of the main wiring1 begins to rise. As a result, thermal stress is generated in each ofthe main wiring 1, a barrier film 3 and insulator layers 2, 4 and 5 onthe basis of a difference between linear expansion coefficients thereof.

In the electric fuse structure of the embodiment 1, the linear expansioncoefficient of the insulator layer 4 is considerably lower than that ofthe main wiring 1. For this reason, the degree of the expansion of theinsulator layer 4 is smaller than that of the main wiring 1. Theinsulator layer 4 is brought into contact with the main wiring 1.Accordingly, even if the main wiring 1 is to expand, the insulator layer4 restrains the expansion. As a result, tensile force is generated inthe upper portion of the main wiring 1 and compressive force isgenerated in the lower portion of the insulator layer 4, as illustratedin FIG. 21, so that stress concentration is generated in encircledportions illustrated in FIG. 21.

When the temperature of the main wiring 1 further rises, the metalconstituting the main wiring 1 changes from the solid to a liquid. Inshort, the metal undergoes phase change. In this way, the volume of themain wiring 1 further increases. At this time, the expansion of the mainwiring 1 is limited by the barrier film 3. For this reason, the mainwiring 1 expands only upwards, as represented by white arrows eachsurrounded by a black line in FIG. 22, whereby the insulator layer 4 ispushed upwards.

On the basis of a synergistic effect of the matter that stressconcentration is generated at both ends of the upper portion of the mainwiring 1 before the main wiring 1 is liquefied and that the insulatorlayer 4 is pushed upwards, cracks 6 are generated in the insulatorlayers 4 and 5 from the points where the stress concentration isgenerated, the points functioning as starting points.

By the generation of the cracks 6, a cavity is generated in theinsulator layer 4. The width of the cavity is very small. The mainwiring 1 is liquefied, and thus the liquefied main wiring 1 is absorbedinto the cracks 6 by a capillary phenomenon. As a result, in the mainwiring 1, discontinuous portions are formed at positions different fromthe positions where the cracks 6 are generated.

In FIGS. 23 to 32, a series of states that the cutting of the electricfuse 10 progresses as described above are illustrated in succession withtime. As the number of one out of these figures is larger than others,the state illustrated in the figure is a state which makes itsappearance later. FIGS. 23, 25, 27, 29 and 31 are each a top view, andFIGS. 24, 26, 28, 30 and 32 are each a sectional view.

As illustrated in FIGS. 31 and 32, when a predetermined amount of theliquefied main wiring 1 is absorbed into the cracks 6 by a capillaryphenomenon, the main wiring 1 and the barrier film 3 are cut. Thebarrier film 3 is cut by force generated when the main wiring 1 isabsorbed. Even if residues of the barrier film 3 slightly remain at thistime, the barrier film 3 can be cut without failure by causing a verysmall electric current to flow into the main wiring 1 continuously. InFIGS. 33 and 34, the electric fuse 10 having an actual cut portion 1000is illustrated.

When the electric fuse 10 is cut by use of a capillary phenomenon asdescribed above, no crack is generated in the insulator layer 2 belowthe main wiring 1. Moreover, when the electric fuse 10 is heated to atemperature that is slightly higher than the melting point of the mainwiring 1, the electric fuse 10 can be cut. It is therefore possible toprevent a thermally adverse effect from being produced on surrounds ofthe electric fuse 10 and prevent elements, such as a transistor, fromdamaging the formed semiconductor substrate SC.

Embodiment 2

With reference to FIGS. 35 and 44, a method of an embodiment 2, whereinthe resistance of an electric fuse is increased, is described herein.The electric fuse structure used in the embodiment 2 may be the same asin the embodiment 1.

In the embodiment 2, a method for cutting the electric fuse 10 describedin the embodiment 1 more certainly is described. Specifically, describedis a matter that it is necessary to adjust the rise time of electricpulses caused to flow into the electric fuse 10 in order to cut theelectric fuse 10 more certainly.

When the electric fuse is cut, the temperature of the main wiring 1needs to reach the melting point or a higher temperature. However, aphenomenon generated when the electric fuse 10 is cut is varied inaccordance with the period from a time when a rise in the temperature ofthe main wiring 1 starts to a time when the temperature of the mainwiring 1 reaches the melting point or a higher temperature. Accordingly,unless this period is adjusted, it is impossible to cut the electricfuse without damaging surrounds of the electric fuse 10.

FIG. 35 shows two kinds of electric current pulses which have the samevalues of currents flowing into the electric fuse 10 but have differentrise times and falls times. As illustrated in FIG. 35, the electriccurrent pulse shown as a proper pulse has a far shorter fall time (i.e.,a period from a time when the supply of an electric current is startedto a time when the supply of an electric current having a constant valuestarts) than the electric current pulse shown as an improper pulse.

FIG. 36 shows a state of the electric fuse 10 which is cut by receivingthe supply of electric current pulses as improper pulses as illustratedin FIG. 35, and a state of the electric fuse 10 which is cut byreceiving the supply of electric current pulses as proper pulses asillustrated in FIG. 35.

As described above, the method of increasing the resistance of theelectric fuse 10 in the embodiment 1, in particular, the method ofcutting the electric fuse 10 is a method of generating the cracks in theinsulator layer 4 to cause the liquefied main wiring 1 to be absorbedinto the cracks 6, thereby cutting the main wiring 1. However, if theinsulator layer 4 is softened by Joule heat from the main wiring 1, thecracks 6 are not generated in the insulator layer 4; therefore, theelectric fuse 10 may not be cut in a short time. If in this case anelectric current is caused to flow into the electric fuse 10 for a longtime so that heat is continuously generated from the electric fuse 10over a long time, the surrounding structure of the electric fuse 10 maybe damaged.

Thus, the shape of electric current pulses for generating the cracks 6in the insulator layer 4 to cut the electric fuse 10 in a short timewill be discussed hereinafter.

First, considered is a rise in the temperature of a metallic cube havingthe same volume as the electric fuse 10 when the cube is uniformlyheated in an adiabatic state. The reason why this matter is consideredis as follows: it can be estimated that the electric fuse 10 is presentin a state equivalent to an adiabatic state since the fuse 10 issurrounded by the insulator layers 2 and 4.

TABLE 4 Melting Wiring point Specific Melting Heat of BoilingEvaporation Electric Volume film Wiring Wiring arrival heat pointmelting point heat current resistivity thickness width length Densitytime Material (kJ/(kgK)) (° C.) (kJ/kg) (° C.) (kJ/kg) (mA) (×10⁻⁸ Ωm)(μm) (μm) (μm) (kg/m³) (μs) Al 1 660.4 311.3 2486 10888.9 15 2.7 0.2 0.18 2690 0.113 Cu 0.47 1084.5 213 2580 4789 15 1.6 0.2 0.1 8 8500 0.470 Al1 660.4 311.3 2486 10888.9 30 2.7 0.2 0.1 8 2690 0.028 Cu 0.47 1084.5213 2580 4789 30 1.6 0.2 0.1 8 8500 0.118 Calculation conditions wiringwidth: 0.1 μm, wiring thickness: 0.2 μm, wiring length: 8 μm, wiringvolume: 0.16 μm³, and applied current: 15 mA, and 30 mA.

Herein, a case is considered where electric current pulses which have acurrent value of 15 mA and 30 mA, respectively, and each have a risetime of 0 μs are each supplied to the metallic cube. The electric pulsesare theoretical pulses. The time required until each of the metals isliquefied in this case is shown in Table 4.

The melting point arrival time of each of the metals shown in Table 4 isthe shortest time ts necessary until the cube of the metal is liquefied.When the value of the current supplied to the cube of Cu is, forexample, 15 mA, the shortest time ts, which is necessary until the cubeis liquefied, is about 0.5 μs. When the value of the current supplied tothe cube of Cu is 30 mA, the shortest time ts is about 0.1 μs.

Since the shortest time ts is a time necessary until the cube of a metalreaches the melting point thereof, the time ts does not preciselyrepresent a time required for a rise in the temperature of the electricfuse 10, which is a long and thin line. Since the electric currentpulses given to the cube are theoretical pulses which do not have anyrise time (rise time=0 μs), the pulses are different from electriccurrent pulses having a rise time.

FIG. 37 shows a relationship between the rise time of electric pulses (2μs) and the ratio of the resistance of the electric fuse 10 after thefuse 10 is cut to that of the electric fuse 10 before the fuse 10 iscut. As can be understood from FIG. 37, when the current value of theelectric current pulses is 15 mA and the rise time thereof is 0.5 μs,the electric fuse 10 is cut. However, when the current value of theelectric current pulses is 15 mA but the rise time thereof is over 0.5μs, the resistance of the electric fuse 10 hardly increases. FIG. 37shows results from an experiment wherein electric current pulses weregiven to the actual electric fuse 10.

From the comparison of the experimental results shown in FIG. 37 withthe values estimated theoretically from the use of the values shown inTable 4, it is understood that the above-mentioned shortest time ts canbe adopted as an index for deciding the rise time of actual pulsessupplied to the electric fuse 10 which is long and thin and is actuallyused. In other words, it can be considered that when the rise time ofelectric current pulses given to the actual electric fuse 10 is shorterthan the theoretically-estimated shortest time ts, the electric fuse 10can be properly cut.

When it is assumed that the rise time, a time when a constant electriccurrent is caused to flow, and the fall time are equal to each other(tm) under consideration of the above-mentioned matters, the cut time ofthe electric fuse 10 can be represented by the following expression:

Cut time=[rise time]+[time when a constant electric current is caused toflow]+[fall time]=3×[shortest time (ts)]

It can be understood from this expression that when an electric currentof 15 mA is caused to flow into the electric fuse 10, the electric fuse10 can be cut in a time of 1.5 μs or less.

When the rise time is actually shorter, the following can be admittedeven if the width and the thickness of the main wiring 1 are scattered:the adjustment of the time when the constant electric current is causedto flow makes it possible to cut the electric fuse 10 in a time of lessthan 1 μs.

According to the electric fuse cutting method of the embodiment 2, theelectric fuse 10 can be cut in a time of about several microseconds.Specifically, according to the electric fuse cutting method of theembodiment 2, the electric fuse 10 can be cut in a very short time whichis 1/133 (=1.5 μs/200 μs) of the time required for cutting an electricfuse in the above-mentioned conventional electric fuse cutting method.

However, when the electric fuse 10 illustrated in FIGS. 5 and 6, whichhas only a linear shape, is cut by the above-mentioned method, eitherone of sites at both sides of the site where the capillary phenomenon isgenerated is cut. However, the position of the cut portion 1000 cannotbe specified. FIGS. 38 and 39 show examples of positions of a crack 6generated when an electric fuse 10 made only of a linear shape is cutand examples of the position of a cut portion 1000.

It is theoretically known that when the length of the electric fuse 10is 12 μm, the crack 6 is generated at a position 6.6 μm apart from oneof ends of the electric fuse 10 and the cut portion 1000 is formed at aposition 5.1 μm apart from the end.

It is also understood from FIG. 39 that most of the examples of theposition of the cut portion 1000 are positioned upstream from theexamples of the position of the crack 6 but about measurement resultseach surrounded by an ellipse, which are different from the othermeasurement results, the examples of the position of the cut portion1000 are positioned downstream from the examples of the position of thecrack 6. It appears that this tendency is produced regardless of thelength of the electric fuse 10. As described herein, when the electricfuse 10 made only of a linear shape is used, there arises a problem thatthe position of the cut portion 1000 is not easily specified.

One method for solving this problem is a method of generating cracks 6at two sites, and causing the melted main wiring 1 to be absorbed intoeach of the two sites, thereby cutting the electric fuse 10 at aposition between the two sites. For this method, it is effective to usethe electric fuse 10 having a meandering shape as illustrated in FIGS. 3and 4, that is, the electric fuse 10 having bent portions 10 c andlinear portions 10 d.

According to such an electric fuse, which has a meandering shape, suchas an electric fuse 10 illustrated in FIGS. 40 and 41, stressconcentration can be generated at positions 2000 which are each near oneof bent portions 10 c of the electric fuse 10. For this reason, theposition of a cut portion 1000 can be specified. In other words, the cutportion 1000 can be formed at a position between the two bent portions10 c.

However, when the distance S between linear portions 10 d illustrated inFIG. 42 is small, cut pieces are scattered so that the cut linearportions 10 d of the electric fuse 10 may short-circuit, as illustratedin FIG. 43.

It is known, from consideration of diffusion of the cut portion 1000 tothe outside of a barrier film 3, whether or not the cut linear portions10 d of the electric fuse 10 short-circuit depends basically on the sizeof the cut portion 1000. The size of the cut portion 1000 is about lessthan 0.3 μm; therefore, it is desired that the distance S between thelinear portions 10 d of the electric fuse 10, which has the meanderingshape, is 0.3 μm or more. In short, it is desired that the distance Sbetween the linear portions 10 d near the cut portion 1000 is largerthan the size of the cut portion 1000. As illustrated in FIG. 44, thedistance S between linear portions 10 d is 0.3 μm or more; in order togenerate stress concentration easily, the distance S0 between linemoieties near each bent portion 10 c is desirably smaller than thedistance S between the linear portions 10 d.

Embodiment 3

With reference to FIGS. 45 to 47, a method of an embodiment 3, whereinthe resistance of an electric fuse is increased, is described herein.The electric fuse structure in the embodiment 3 may be the same as inthe embodiment 1.

In the case of using the method of increasing the resistance of anelectric fuse according to each of the embodiments 1 and 2, the cracks 6may not extend immediately in the insulator layer 4. This would bebecause a considerable large electric current cannot be caused to flowinto the electric fuse 10 because of a problem resulting from thestructure of the circuit and thus thermal stress generated in theelectric fuse structure is not sufficiently large for generating thecracks 6. For this reason, the electric fuse 10 may not be cut by thecutting method described as the embodiment 1 or 2. Accordingly, a methodfor cutting the electric fuse 10 certainly in this case will bedescribed hereinafter.

When an electric current is caused to flow into the electric fuse 10,the main wiring 1 changes from solid to liquid as the temperature of theelectric fuse 10 rises. When no crack is generated in the insulatorlayer 4, an electric current flows into the main wiring 1 in the liquidstate. When an electric current of 10⁸ A/m² or more is caused to flowinto the main wiring 1 in this case, electromagnetic force is generatedtoward the central of the main wiring 1. This is called pinch effect. Asa result, a liquefied portion in the main wiring 1 will be shrunken bysurface tension and the pinch effect. This pinch effect will bedescribed in detail hereinafter.

For simplicity of the description, it is presumed that the main wiring 1has a columnar shape. When an electric current flows into the mainwiring 1, a magnetic field is formed so that Lorentz force F isgenerated in a direction perpendicular to the direction along which theelectric current flows. At this time, the magnetic field B isrepresented by the following equation (1):

$B = \frac{\mu \; 0 \times {\int{i \cdot {s}}}}{2\; \pi \; r}$

When the radius of the above-mentioned column is represented by r (m),the magnetic field B (A/m) and the density j (A/m²) of the current areused to represent the Lorentz force F (N/m³) generated in each unitvolume of the main wiring 1 by the following equation (2):

$\begin{matrix}{F = {j \cdot \frac{\mu \; 0 \times {\int{i \cdot {s}}}}{2\; \pi \; R}}} \\{= {\mu \; {0 \cdot \frac{I}{\pi \; R^{2}} \cdot \frac{I}{2\; \pi \; R}}}}\end{matrix}$

In the equation (1), it is presumed that the current density j isuniform. In the formula (1), μ0 is the magnetic permeability, S is anyclosed surface, I is the value of the current given to the main wiring1, and R is the distance from the portion which constitutes the mainwiring 1 to the center of the column. When the density of the materialwhich constitutes the main wiring 1 is represented by ρ (kg/m³), theacceleration a generated in each unit volume of the main wiring 1 by theLorentz force F is equal to F/ρ (m/s²).

Accordingly, using the acceleration a, the time t (s) when the distancebecomes zero, that is, the time when the electric fuse 10 becomestheoretically narrowest is represented by t=√{square root over ()}(2r/a).

When it is presumed that the radius r of the main wiring 1 is 0.075 μm,the applied current is 15 mA, the density ρ is 8780 kg/m³, and themagnetic permeability μ0 is 1.256637×10⁻⁶ (H/m), the Lorentz force F,the acceleration a, and the time t are calculated as follows:

F=3.3953×10¹⁰ N/m³,

a=3.8671×10⁶ m/s²′ and

t=197 ns.

It can be considered from the above-mentioned matter that when pincheffect is used, the time (t) necessary for making the main wiring 1narrowest becomes very short. In other words, it is expected that evenif the width of given electric current pulses is small, the diameter ofthe electric fuse 10 becomes very small by pinch effect. The currentdensity j is 8.49×10¹¹ A/m².

In order to use pinch effect to cut the electric fuse 10, the supply ofthe electric current (pulses) to the electric fuse 10 is stopped whenthe liquefied portion of the electric fuse 10 becomes narrowest, thatis, the time t when the above-mentioned R becomes zero. From this time,the solidification of the main wiring 1 starts. When the supply of theelectric current (pulses) to the electric fuse 10 is stopped, retainingforce acts in a direction opposite to the direction along which theelectric fuse 10 is shrunken. As a result, the electric fuse 10 startsto swell.

When electric current pulses are again supplied to the main wiring 1, aphenomenon that the above-mentioned shrinking force and retaining forceare alternately generated is repeated, so that the diameter of themoiety onto which the Lorentz force L of the main wiring 1 acts becomessmaller. Accordingly, at last, the liquefied portion of the main wiring1 is cut. FIG. 45 illustrates steps of repeating switching-on andswitching-off of electric current pulses so as to form a cut portion1000 on the electric fuse 10.

In the method of cutting an electric fuse according to the embodiment 3,shrinking force (Lorentz force L) generated by switching-on of anelectric current on the basis of pinch effect and force (retainingforce) generated in a swelling direction by switching-off of theelectric current act alternately and repeatedly onto the electric fuse10. Since the main wiring 1 is liquefied at the position where the pincheffect is generated, surface tension is also generated together with theLorentz force F. At this time, the insulator layers 2, 4 and 5 aroundthe electric fuse 10 are softened by heat from the electric fuse 10.Thus, the electric fuse 10 swells outside. As a result, a centralportion of the electric fuse 10 gradually becomes hollow. At last, theelectric fuse 10 is cut. The liquefied electric fuse 10 is easily stayedat the lower side thereof by gravity. Thus, the cutting of the electricfuse 10 starts from the upper side thereof.

As described above, in the method of cutting an electric fuse accordingto the embodiment 3, a predetermined electric current pulse isrepeatedly given to the electric fuse 10, whereby pinch effect isrepeatedly generated. As a result, the electric fuse 10 is cut at itscut portion 1000, as illustrated in FIG. 46.

According to the method of cutting an electric fuse according to theembodiment 3 also, the time required until the main wiring 1 isliquefied and the time when an electric current (pulses) is caused toflow into the main wiring 1 are very short; therefore, thermal damagegenerated around the electric fuse 10 is restrained.

In the method of cutting an electric fuse according to the embodiment 3,for example, the temperature of the electric fuse 10 is kept at 1200° C.only for 5 μs. In this case, moieties where the temperature becomes 600°C. or higher in the insulator layers 2, 4 and 5 arranged around theelectric fuse 10 are moieties wherein the distance from the electricfuse 10 is less than 0.4 μm. Accordingly, an adverse effect based onheat generated from the electric fuse 10 is hardly produced onto anyelement arranged around the electric fuse 10.

A theory and experimental results have demonstrated that when theelectric fuse 10 is cut by pinch effect, a central portion of the fuse10, which has equal distances from both ends of the fuse 10, is cut.

It should be understood that all the embodiments disclosed herein areillustrative and are not restrictive. The scope of the present inventionis specified not by the above-mentioned description but by the appendedclaims. All modifications which have meanings equivalent to the claimsor which are within the scope recited in the claims are intended to beincluded in the invention.

1. A semiconductor device, comprising: an insulator layer; and an electric fuse which is formed in the insulator layer, and has a larger linear expansion coefficient than that of the insulator layer, and further has a lower melting point than that of the insulator layer.
 2. The semiconductor device according to claim 1, wherein the electric fuse comprises: a main wiring; and a barrier film which contacts with each of the main wiring and the insulator layer, wherein the linear expansion coefficient of the barrier film is smaller than that of the main wiring and is larger than that of the insulator layer, and wherein the melting point of the barrier film is higher than that of the main wiring and is lower than that of the insulator layer.
 3. The semiconductor device according to claim 2, wherein the main wiring comprises copper, aluminum or iron.
 4. The semiconductor device according to claim 2, wherein the barrier film comprises a tantalum film.
 5. The semiconductor device according to claim 2, wherein the barrier film comprises: a first tantalum film which contacts with the insulator layer; a tantalum nitride film which contacts with the first tantalum film; and a second tantalum film which contacts with the tantalum nitride film and the main wiring.
 6. The semiconductor device according to claim 1, wherein the insulator layer comprises a first insulator layer having a trench in which the electric fuse is formed, and a second insulator layer formed over the first insulator layer and the electric fuse.
 7. The semiconductor device according to claim 6, wherein the second insulator layer comprises a SiCN film, a SiN film, a bi-layered structure film having a SiCN film and a SiN film, or a low-k film having a dielectric constant of 3 or less.
 8. The semiconductor device according to claim 1, wherein a cap film having a higher electric resistance than that of the main wiring is formed between the main wiring and the insulator layer.
 9. The semiconductor device according to claim 1, wherein the electric fuse is surrounded by an electroconductive material floating electrically.
 10. A semiconductor device, comprising: a semiconductor substrate; a gate electrode formed over the semiconductor substrate; an interlayer dielectric covering the gate electrode; a fine layer formed over the interlayer dielectric; a semiglobal layer formed over the fine layer; a global layer formed over the semiglobal layer; and an electric fuse formed in at least one selected from the fine layer, the semiglobal layer, and the global layer.
 11. A semiconductor device, comprising: an insulator layer; and an electric fuse which is formed in the insulator layer, and has a meandering shape comprising a linear portion and a bent portion, the distance between moieties near the bent portion being smaller than the distance between moieties other than the moieties near the bent portion.
 12. A method of increasing the resistance of an electric fuse according to a semiconductor device which comprises an insulator layer; and an electric fuse which is formed in the insulator layer, and has a larger linear expansion coefficient than that of the insulator layer, and further has a lower melting point than that of the insulator layer, the method of increasing the resistance of the electric fuse comprising the steps of: supplying an electric current to the electric fuse, thereby melting the electric fuse and further generating a crack in the insulator layer; and after the step above, using a capillary phenomenon to cause a part of the melted electric fuse to be absorbed into the crack, thereby forming a discontinuous portion in the electric fuse.
 13. The method according to claim 12, wherein the electric current is supplied as pulse waves to the electric fuse, and the rise time of the pulse waves is adjusted, thereby generating the crack.
 14. A method of increasing the resistance of an electric fuse according to a semiconductor device which comprises an insulator layer; and an electric fuse which is formed in the insulator layer, and has a larger linear expansion coefficient than that of the insulator layer, and further has a lower melting point than that of the insulator layer, the method of increasing the resistance of the electric fuse comprising the steps of: supplying an electric current to the electric fuse, thereby making the electric fuse narrow by use of pinch effect; and stopping the supply of the electric current, thereby forming a cavity in the electric fuse by use of retaining force of the electric fuse.
 15. The method according to claim 14, wherein the step of supplying the electric current and the step of stopping the supply of the electric current are alternately repeated. 